This is standard behavior. When a filesystem is created, some of the available space is held in reserve, and is available only to the superuser. This provides a margin of error when you accidently fill the disk, but more importantly, it helps keep disk fragmentation to a minimum. I'm not sure about the default for FreeBSD, but the default for OpenBSD is 5%, so you could see up to 105% in df(1) output on that OS.

I figured out the reason that the disk is actually slightly smaller than it was before - I fiddled around with the partition sizes to give a little more to Swap, so that's why the backup is slightly too large.